1. Field of the Invention
The present invention relates to a low-voltage power supply circuit for illumination, an illumination device, and a low-voltage power supply output method for illumination, and more particularly to a low-voltage power supply circuit for illumination, an illumination device, and a low-voltage power supply output method for illumination that uses a delighted light source such as an organic EL or LED.
2. Description of the Related Art
The development of high-luminance LEDs and organic ELs is currently progressing and these devices will soon find use for the purpose of illumination. Although high-luminance LEDs and organic ELs still lack the luminous efficacy of fluorescent lamps, they are said to offer smaller size, thinner construction, and longer life, and above all, enable elimination of the use of mercury, and therefore hold promise as a light source for illumination.
Both high-luminance LEDs and organic ELs are dc-driven elements and emit light by means of the flow of dc current in these dc drive elements. As a result, in order to use a residential ac power supply to cause these dc-driven elements to emit light requires a power supply that converts an ac power supply to a dc power supply. In addition, high-luminance LEDs and organic ELs are devices that emit light with stability by means of the flow of a constant current and therefore necessitate a circuit for limiting current. Unless the luminous efficacy of these dc-driven elements is dramatically improved, the use of these dc-driven elements as illumination devices requires power on the order of 50-200 W.
A high-power illumination device must be provided with a power-factor improvement circuit. In the prior art, the power-factor improvement circuit that is typically used is of the booster type. When the power supply is 100V, this power-factor improvement circuit supplies as an output voltage a dc voltage of 200-300V and therefore cannot be used as is for a low-voltage element such as an LED. As a result, the least complex method is to both limit this dc voltage output to a constant current by a current-limiting circuit and reduce the voltage to the drive voltage of the LED to light the LED. However, this solution not only results in an increase in circuit scale but also creates problems for reducing cost.
The power-factor improvement circuit that is used in the prior art is a booster circuit, and the output voltage must therefore be higher than the maximum instantaneous value of the ac power supply voltage VAC. For example, when the power supply voltage is 100V, the output voltage is set to 200V-300V. On the other hand, the forward voltage drop of an LED is 2-4V and the forward voltage drop of an organic EL is as low as 10-20V, and the excessively high output voltage of a power-factor improvement circuit therefore complicates the direct drive of these elements even when a plurality of elements are driven in a series by the power-factor improvement circuit.
Accordingly, examples of the prior art required the insertion of a constant-current circuit in a stage following the power-factor improvement circuit for simultaneously supplying a constant current to the load such as an LED and lowering the high output voltage of the power-factor improvement circuit to the low drive voltage of loads such as LEDs. Accordingly, the prior art entailed the problems of a complex circuit, an increased number of components, and the inability to lower costs.
FIG. 1 is a block diagram showing the circuit configuration of the first example of the prior art. Approximately the left half of FIG. 1 is the power-factor improvement circuit, and approximately the right half of FIG. 1 is the constant-current circuit. In addition, FIG. 2a is a block diagram of the power-factor control circuit shown in FIG. 1, and FIG. 2b is a block diagram of the current control circuit shown in FIG. 1. FIGS. 3a-3f are waveform charts for explaining the operation of FIGS. 1, 2a, and 2b. 
The principle components of the power-factor improvement circuit of FIG. 1 are: diode bridge 1, transformer T1, switch element Q1, power-factor control circuit 2a for controlling this switch element Q1, and output filter 3. This power-factor improvement circuit controls the phase of AC power supply voltage VAC (FIG. 3a) and power supply current IAC to improve the power factor. Output voltage 7 of the power-factor improvement circuit is supplied to the constant-current circuit that is approximately the right half of FIG. 1, and the LED current ILED that flows to the LED of load 6 is controlled to a constant value.
FIG. 2a is a block diagram for explaining the details of power-factor control circuit 2a shown in FIG. 1. This power-factor control circuit 2a is made up from: multiplier 11, reference power supply 12a, error amplifier 14a, comparator 16a, driver 17a, zero-current detector 18, and flip-flop 19.
Output V7 of the power-factor improvement circuit is fed back to power-factor control circuit 2a of the control IC as output partial voltage V3 (FIG. 3c) that has undergone voltage division by resistor R5 and resistor R6. This output partial voltage V3 is compared with a reference voltage of reference power supply 12a at error amplifier 14a, and the difference is amplified and applied to one of the input terminals of multiplier 11. Voltage V2 (FIG. 3b), which is obtained by subjecting VAC, which is the AC input, to full-wave rectification by diode bridge (D1) and then voltage-division to an appropriate value by resistor R1 and resistor R2, is applied to the other input terminal of multiplier 11. Multiplier 11 generates voltage V4 (FIG. 3d), which is the result of multiplying these voltages, and supplies this result to one terminal of comparator 16a. Accordingly, the output V4 of multiplier 11, is voltage similar to AC power supply voltage VAC and has an amplitude that is proportional to output voltage V7 of power-factor improvement circuit.
Converted voltage V8 (FIG. 3d), which is obtained by converting the current value IQ1 that flows to switch element Q1 to a voltage value by resistor R6, is applied to the other input terminal of comparator 16a. Switch element Q1 turns ON during the interval from the time that the current IT1 that flows to transformer T1 becomes “0” to the time that converted voltage V8 reaches the level of multiplied voltage V4. During this time interval, the current increases substantially linearly, but the proportion of this increase is determined by the primary inductance of transformer T1 and the instantaneous value of power supply voltage VAC.
When the above-described ON interval ends and switch element Q1 turns OFF, the current that flows to switch element Q1 becomes “0” instantaneously and a sawtooth wave is produced, but after the attenuated current that is determined by the primary inductance flows to the primary coil of transformer T1 for a certain interval, a current flows that becomes “0” (IT1 of FIG. 3e). This transformer T1 also implements zero-current detection, and at the same time, has the function for converting energy (i.e., conversion of voltage) as the inductance of a booster chopper circuit.
By repetition of this process, an interrupted current having a triangular wave flows to the primary coil of transformer T1. By selecting components to achieve a frequency sufficiently higher than the frequency of VAC, the high frequency of voltage V8 is normally 20-200 kHz.
The output of comparator 16a is supplied to the reset terminal of flip-flop 19. This flip-flop 19 sets switch element Q1 to ON during the interval that it is set. The above-described voltage V4 and voltage V8 are compared by this comparator 16a, and when voltage V8 surpasses voltage V4, the output of comparator 16a inverts, flip-flop 19 is reset, and switch element Q1 turns OFF.
At the instant switch element Q1 turns OFF, counter-electromotive force is generated at the primary coil of transformer T1, passes through diode D3 and charges capacitor C3. During the interval that this charge current flows, current IT1 that gradually attenuates continues to flow to the primary coil of transformer T1 even after switch element Q1 turns OFF.
The change to “0” of current IT1 that flows to the primary coil of transformer T1 is detected by the secondary coil of transformer T1 and zero-current detector 18. Upon detecting that current IT1 has become “0,” zero-current detector 18 resets flip-flop 19, whereby switch element Q1 turns ON.
Through the repetition of the above-described operations, the phase of the average value of current IT1 that flows to the primary coil of transformer T1, i.e., power supply input current IAC, becomes equal to the phase of AC power supply voltage VAC (FIG. 3f), and the power factor is controlled to substantially “1.”
In addition, because its output voltage V7 is fed back to power-factor control circuit 2a, the output voltage V7 of power-factor control circuit 2a is controlled to a substantially constant value, the size of this output voltage V7 normally being set to 200-300V when the AC power supply voltage is 100V.
In addition, the constant-current circuit portion is made up from the widely used chopper-type step-down circuit, and is made up from: current control circuit 7, switch element Q2, and output filter 3. FIG. 2b is a block diagram for explaining the details of current control circuit 7 shown in FIG. 1. This current control circuit 7 is made up from: reference power supply 22, error amplifier 23, sawtooth-wave oscillator 21, comparator 24, and driver 25.
Current control circuit 7 detects the load current as voltage V9 by means of resistor R4, and applies this current to one terminal of error amplifier 23. The reference voltage from reference power supply 22 is applied as input to the other terminal of error amplifier 23. The output of this error amplifier 23 is compared with the output of sawtooth-wave oscillator 21 in comparator 24, and the output of comparator 24 is supplied as output by way of driver 25 to drive switch element Q2.
This switch element Q2 is a chopper-type step-down circuit. Current control circuit 7, by feeding back voltage V9 that is a voltage obtained by converting load (LED) current ILED by resistor R4, maintains LED current ILED at a constant value and simultaneously supplies a low voltage appropriate for driving an LED.
As described in the foregoing explanation, the circuit of the first example of the prior art inserts a constant-current circuit in a stage following the power-factor improvement circuit, steps down the high output voltage, and supplies a constant current to a load such as an LED. As a result, the formation of this circuit requires high withstand-voltage components such as the switch elements, diodes, coils, and large-scale capacitors, and the device consequently has the drawback of large size. In other words, this device entails the problems of complex circuit, increased number of components, and the inability to lower costs.
The second example of the prior art is the discharge lamp lighting device disclosed in WO2001-60129. This discharge lamp lighting device simplifies the output circuit and is shown in the block diagram of FIG. 4. This discharge lamp lighting device is made up from: diode bridge 1a, step-up/step-down converter 31, polarity switching circuit 32, start pulse generation circuit 33, control power supply circuit 34, and control unit 35. Diode bridge 1a implements full-wave rectification of commercial AC, step-down/step-up converter 31 steps-up and steps-down the voltage that has undergone full-wave rectification, and polarity switching circuit 32 is composed of switch elements Q5a-5d and switches the polarity of current that flows to discharge lamp 6a. In addition, start pulse generation circuit 33 generates high-voltage pulses to start the discharge lamp of load 6a. 
Step-up/step-down converter 31 is made up from: switch element Q2, transformer T1, diode D2, and capacitor C2. Control unit 35 is made up from: detection circuit 41 for detecting the zero-cross of commercial AC, control circuit 42 for controlling step-up step-down converter 31, current detection circuit 43 for detecting the current of the discharge lamp by means of current detection resistor R4, start pulse control circuit 44 for controlling start pulse generation circuit 33, target current calculation circuit 45, and polarity switch control circuit 45 for controlling polarity switch circuit 32.
Explanation next regards the operation of this discharge lamp lighting device. First, when power is supplied from a commercial ac power supply, control power supply circuit 34 generates and supplies a control power supply for control unit 35, whereby control unit 35 begins operation. In control unit 35, start pulse control circuit 44 controls start pulse generation circuit 33 and applies a high-voltage pulse to the discharge lamp to light discharge lamp 6a. 
When discharge lamp 6a lights up, current begins to flow to current detection resistor R4, and current detection circuit 43 detects this current. On the other hand, a target current is calculated in target current calculation circuit 45. Polarity switch control circuit 46 here compares the current that has been detected by current detection circuit 43 with the target current that has been calculated by target current calculation circuit 45, controls step-up/step-down converter 31 such that the detected current equals the target current, and controls feedback.
In step-up/step-down converter 31, switch element Q1 repeatedly turns ON and OFF at a high frequency of several tens of kHz, whereby current flows to the primary side of transformer T1 when switch element Q1 is in the ON state and energy is accumulated in transformer T1. On the other hand, when switch element Q1 is in the OFF state, the accumulated energy is discharged as power to the secondary side of transformer T1. The discharged power is a high frequency of several tens of kHz, and the high-frequency component is eliminated by diode D2 and capacitor C2 and supplied to the discharge lamp.
When the detected current of current detection circuit 43 is lower than the target current of target current calculation circuit 45, converter control circuit 42 increases the time interval of the ON state of switch element Q1 to increase the power that is discharged to the secondary side, whereby the current that flows to discharge lamp 6a increases. On the other hand, when the detected current is greater than the target current, converter control circuit 42 reduces the time interval of the ON state of switch element Q2, whereby the power that is discharged to the secondary side is decreased and the current that flows to discharge lamp 6a drops. By implementing these operations at high speed, control is effected such that the current of the discharge lamp matches the target current.
Polarity switch control circuit 46 next controls polarity switch circuit 32 such that the set of switch elements Q3a and Q3d and the set of switch elements Q3c and Q3b alternately turn ON, whereby the dc current that is supplied as output from step-up/step-down converter 31 is converted to an alternating current and flows to the discharge lamp. Detection circuit 41 here supplies a zero-cross detection signal when zero-volts is attained in the periodic change of the voltage in the commercial ac power supply.
Target current calculation circuit 45 receives the zero-cross detection signal from zero-cross detection circuit 41, and calculates the target current such that the target current value becomes small in the vicinities of 0° and 180° and the target current value becomes great in the vicinities of 90° and 270° with respect to the commercial ac voltage waveform. Control unit 35 receives the zero-cross detection signal from detection circuit 41, and switches the set of switch elements 5a and 5d that switch between the ON state and OFF state and switches the set of switch elements 5c and 5b that switch between the ON state and the OFF state.
In this way, the polarity of the current that flows to discharge lamp 6a switches at 0° and 180° to produce a sinusoidal current synchronized with the commercial ac power supply VAC. The current that flows from commercial ac power supply VAC to the discharge lamp lighting device and the current that flows to discharge lamp 6a are in a proportional relation, whereby the input current of the discharge lamp lighting device is also a sinusoidal current synchronized to the commercial ac power supply, and the input power factor is increased. In addition, because a power-factor improvement circuit such as a booster inverter is not required, a compact and inexpensive discharge lamp lighting device can be obtained.
However, power of 50-200 W was required for use as an illumination device in the above-described first example of the prior art. An illumination device of this level of power requires a power-factor improvement circuit. The output of this power-factor improvement circuit further becomes a constant current in the current limiting circuit, but as previously explained, this results in increased circuit scale and presents an obstacle to lowering costs.
In response to these problems, the present invention investigates the feasibility of providing a current-limiting capability to the power-factor improvement circuit. If this method is adopted, the time constant of the feedback of current that flows to a light-emitting device must be made sufficiently greater than the period of the ac power supply, and this requirement has the drawback of preventing following in the event of sudden changes in the current that flows to the light-emitting device. In addition, the ripple component of the ac power supply is carried by the light-emitting device current and therefore cannot be avoided, with the resulting drawback that a certain degree of luminous ripple occurs. Neither of these drawbacks occurs in a method in which a current control circuit is provided separately.
Although a lamp lighting device with a simplified output circuit was disclosed in the above-described second example of the prior art, this is a circuit for lighting a discharge lamp and therefore serves as an ac lighting device in which the polarity of the current that flows to the discharge lamp is switched by a polarity switching circuit. As a result, the switching of polarity must be implemented in synchronization with the frequency of the commercial power supply in order to improve the power factor, which is the chief objective, and the polarity switching is therefore an indispensable constituent technology. As a consequence, this device cannot be used as a device directed toward lighting an LED or organic EL that is a dc-driven element.